This invention relates generally to spinal surgery and, in particular, to spinal cages of the type used in fusing adjacent vertebrae.
Eighty-five percent of the population will experience low back pain at some point. Fortunately, the majority of people recover from their back pain with a combination of benign neglect, rest, exercise, medication, physical therapy, or chiropractic care. A small percent of the population will suffer chronic low back pain. The cost of treatment of patients with spinal disorders plus the patient""s lost productivity is estimated at 25 to 100 billion dollars annually.
Seven cervical (neck), 12 thoracic, and 5 lumbar (low back) vertebrae form the normal human spine. Intervertebral discs reside between adjacent vertebra with two exceptions. First, the articulation between the first two cervical vertebrae does not contain a disc. Second, a disc lies between the last lumbar vertebra and the sacrum (a portion of the pelvis).
Motion between vertebrae occurs through the disc and two facet joints. The disc lies in the front or anterior portion of the spine. The facet joints lie laterally on either side of the posterior portion of the spine. The osseous-disc combination of the spine coupled with ligaments, tendons, and muscles are essential for spine function. The spine allows movement (flexion, lateral bending, and rotation), supports the body, and protects the spinal cord and nerves.
The disc changes with aging. As a person ages the water content of the disc falls from approximately 85 percent at birth to 70 percent in the elderly. The ratio of chondroitin sulfate to keratin sulfate decreases with age. The ratio of chondroitin 6 sulfate to chondroitin 4 sulfate increases with age. The distinction between the annulus and the nucleus decreases with age. These changes are known as disc degeneration. Generally disc degeneration is painless.
Premature or accelerated disc degeneration is known as degenerative disc disease. A large portion of patients suffering from chronic low back pain are thought to have this condition. As the disc degenerates, the nucleus and annulus functions are compromised. The nucleus becomes thinner and less able to handle compression loads. The annulus fibers become redundant as the nucleus shrinks. The redundant annular fibers are less effective in controlling vertebral motion. The disc pathology can result in: 1) bulging of the annulus into the spinal cord or nerves; 2) narrowing of the space between the vertebra where the nerves exit; 3) tears of the annulus as abnormal loads are transmitted to the annulus and the annulus is subjected to excessive motion between vertebra; and 4) disc herniation or extrusion of the nucleus through complete annular tears. Disc herniation can also cause arthritis of the facet joints which, in turn may cause back pain.
The problems created by disc degeneration, facet arthritis, and other conditions such as spondylolysis, spondylolisthesis, scoliosis, fracture, tumor, or infection are frequently treated by spinal fusion. Such problems may include pain in the back or legs, nerve injury, risk of future nerve injury, or spinal deformity. The goal of spinal fusion is to successfully xe2x80x9cgrowxe2x80x9d two or more vertebrae together. To achieve this, bone from the patient""s body (spine or iliac crest) or from cadavers, is grafted between vertebrae. Alternatively, bone graft substitutes, such as hydroxy appetite and bone morphogenetic protein, may be used. The bone graft is placed between the vertebrae in the disc space and/or over the posterior elements of the vertebrae (lamina and transverse processes). The surgeon scrapes the vertebrae to create bleeding. Blood flows into the bone graft. The scraped bone, blood clot (hematoma), and the bone graft simulates a fracture. As the patient heals, the xe2x80x9cfracturexe2x80x9d causes the vertebrae to be fused and heal together.
Spinal instrumentation, including cages, is placed onto or into the spine to immobilize the vertebrae that are going to be fused. Immobilization leads to a higher fusion rate and speeds a patient""s recovery by eliminating movement. Existing cages are typically hollow metal or carbon fiber devices placed into the disc space. Often cages have treads, grooves, teeth or spikes that engage the vertebral endplates. The hollow center is filled with bone graft. The sides of the cages adjacent to the vertebral endplates contain holes to allow bone growth from one vertebrae through the cage to the adjacent vertebra. The bone graft acts as a bridge for bone growth. Cages immobilize the vertebrae and maintain the separation between the vertebrae, a function of the former disc material. Cages are placed into the disc space after excising a portion of the disc annulus and most of the nucleus. One or two cages may be inserted at each level.
Cages may be placed into the disc space from an anterior or a posterior approach to the spine. Cages may be combined with rods, screws, hooks, or plates. Combining cages with other instrumentation yields a stiffer construct and presumably increases the chance of a successful fusion. If cages are placed from an anterior approach (through the abdomen) the patient must undergo surgery through a second incision over the back (posterior approach) if the surgeon wants to also insert rods and screws. To avoid two incisions and increased patient morbidity, many surgeons prefer to insert the cages from a posterior approach. Rods and screws can be added through the same incision.
The cages currently available for insertion from a posterior approach have a few important weaknesses. Since most cages are inserted into the disc space in their final size and shape, the cages must be large enough to extend from vertebra-to-vertebra. Furthermore, the cages must be wide enough to provide stability and provide adequate surface area for the vertebrae to xe2x80x9cgrowxe2x80x9d together. Large cages inserted posteriorly risk nerve injury (from retracting the nerves to insert the cage or the edge of the cage during insertion) or extensive bone removal. A portion of the vertebra (lamina, a portion of the facet(s) and/or the entire facet(s)) is removed to allow cage insertion. Large cages require more bone removal. Some surgeons remove one or both facet joints to safely insert the cage or cages. Removal of the facet joints destabilizes the spine, which may lead to cage migration and/or pseudoarthrosis (failure of the bone to heal). Either complication may require the patient to undergo additional surgery. Revision procedures are more difficult after removal of the facet joints.
Most of the cages inserted posteriorly have parallel superior and inferior surfaces. The endplates of the vertebrae form the superior and inferior limits of the disc space. The endplates are not typically parallel in the lumbar spine. Generally, the endplates become closer together as one proceeds toward the posterior portion of the disc space. This alignment creates the normal anterior to posterior curvature of the lumbar spine known as lordosis. Cages with parallel superior and inferior surfaces either fit tightly posterior and loosely anteriorly, or require removal of additional endplate posteriorly. Alternatively, the vertebrae can be fused without lordosis. None of these choices are ideal.
This invention improves upon the existing art by providing intervertebral cages, and methods of introducing the same, which require few, if any, structural components within the cage itself, thereby providing a maximum volume for bone-graft filler materials.
According to a first preferred embodiment, a cage according to the invention features a compressed state associated with insertion of the cage into an intervertebral space having an anterior height which is less than or equal to the posterior height, and an expanded state, wherein the anterior height is greater than the posterior height to account for lordosis. Means are further provided for locking the cage into the expanded state once the cage is positioned within the intervertebral space. Such means preferably includes a ratchet mechanism associated with the anterior portion which locks the cage into desired wedge configuration.
Cages according to the invention may passively expand to fill a volume within the disk space, or active expansion means may be provided. In terms of a passive expansion, at least a portion of the frame may be composed of a shape-memory material, causing the cage to naturally expand from the compressed to the expanded state once the cage is positioned within the intervertebral space. Alternatively, superior and inferior members may be attached to the posterior portion so as to form a spring, which causes the cage to naturally expand from the compressed to the expanded state. The superior and inferior surfaces preferably include protrusions for frictionally engaging with vertebral endplates defining the disk space.
In terms of an active expansion, an intervertebral cage according to the invention may include an externally accessible mechanism operative to increase the anterior height once the cage is positioned within the intervertebral space. For example, such a mechanism may include a slug positioned on a threaded rod which moves posterior to anterior when the rod is rotated. An alternative embodiment includes first and second components, each having a compressed state which consumes a compressed volume associated with insertion of the component into an intervertebral space, and an expanded state which consumes a greater volume when the component is positioned within the intervertebral space. In this case, the shape of the components is such that the first component when expanded leaves a gap which is closed by the second component when expanded. In all embodiments, connection means may be provided for interconnecting a plurality of cages together within the same intervertebral space.